BEIRUT, Lebanon — During the fractious weekend debates that ended with the main Syrian exile opposition coalition yielding to international pressure by dropping its refusal to hold peace talks with President Bashar al-Assad’s government, tensions ran so high that one prominent coalition member slapped another in the face, participants in the gathering said.

In the hallways outside the meeting at an Istanbul hotel, young antigovernment activists exasperated with the coalition’s failure to forge an effective opposition said they had grudgingly pressed the group to approve the peace talks, calling them the only hope to slow the killing of Syrians.

Stoking tensions all around, Robert S. Ford, the United States ambassador to Syria, told the activists on the sidelines that the emerging reality presented them with unpalatable options: accept that the current government could continue in power longer than they would like, or face the continued rise of extremist jihadist groups that have terrified residents, clashed with rival insurgents and undermined Western support.

Under intense American, British and European pressure, the coalition voted early Monday, after two days of debate, that it would attend peace talks sponsored by the United States and Russia in Geneva if certain conditions were met, including full access for delivery of humanitarian aid and the release of prisoners.

Although there were “hot debates,” said Radwan Ziadeh, who leads the transitional justice commission in the interim government, “Geneva became an essential option because we’re facing a stalemate between the Free Syrian Army and Assad.”

But while United States officials hailed the step as significant, it risks becoming the latest of many tentative moves toward talks that have proved illusory, since the coalition retained its demand that Mr. Assad play no role in any future political transition, a condition the Syrian government rejects.

The coalition “will lose in both cases,” said Jamil Salo, an antigovernment activist who attended the meeting on the sidelines after fleeing the northeastern city of Raqqa because of threats from an extremist group, Islamic State of Iraq and Syria, or ISIS.

“If they refuse to attend the meeting, they will give legitimacy to the regime, and they will lose the grass roots if they go,” Mr. Salo said in an interview from Istanbul, adding that Mr. Ford had told activists they faced a choice, “ISIS or the regime.”

“We’re not going to Geneva to surrender, but to stop the bloodshed,” said Hisham Marwa, a member of the opposition group, the National Coalition for Syrian Revolutionary and Opposition Forces. “We support the political solution.”

He said the coalition, in a new compromise, was now willing to sit with members of the government as well as people “from inside,” an apparent reference to self-described opposition figures outside the coalition. Those figures, some of whom Russia has sought to include in talks, range from longtime dissidents to former officials whom many Assad opponents see as close to the government.

Independent analysts monitoring the conflict say it is increasingly clear that while a political transition could end with Mr. Assad leaving office, it is increasingly unrealistic to expect his government to agree to that in advance.

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Inside the country, a growing number of people on both sides of the conflict complain that nobody seems to represent their desire to end the war and the suffering it has created, with nine million people forced from their homes and well over 100,000 dead.

Few say they believe that the Geneva talks will yield a solution, yet those talks remain the focus of international diplomacy. Unofficial alternative efforts are underway to jump-start informal talks that could explore the basis for a deal, but they remain in their infancy.

By refusing to go to Geneva, the opposition would hand the government a diplomatic victory. Yet the coalition, already derided by many inside Syria as out of touch, would enrage the most committed of Mr. Assad’s opponents if it agrees to a transitional government that includes him or central government or security figures.

With the Monday vote, the coalition appeared to drop its demand that Mr. Assad step down before any talks. But it continued to insist that it would participate only if there are guarantees that the talks would lead to a full transfer of executive power, including over military and security forces, to a transitional government in which Mr. Assad and “those with blood on their hands” would have “no role.”

The coalition said it would consult fighters inside the country in an effort to win support for attending the Geneva talks. Many insurgent groups have dismissed any participation in the talks as treason; a few have said they are willing to seek a deal to alleviate the country’s suffering.

Kamal Labwani, a member of the coalition who has consistently opposed any talks ahead of Mr. Assad’s ouster, said the meeting had been so tense that the coalition president, Ahmed Jarba, slapped a representative of the rebel Free Syrian Army, Louay Mekdad, during a dispute in which Mr. Mekdad called the voting process reminiscent of Mr. Assad’s Baath Party.

With Mr. Assad’s government holding on to power, the United States has begun saying Mr. Assad has “lost his legitimacy” rather than repeating earlier demands that he step down. Many rebel groups felt betrayed after the August chemical attacks, when a threatened United States military strike on Mr. Assad’s government was called off and instead a deal was made to dismantle Syria’s chemical weapons arsenal.

“Eventually, they will go to Geneva,” Mr. Labwani said of the coalition members. Once there, he said, international leaders “will make fun of them” and use a failed Geneva conference as an excuse “to put Syria under American and Russian control.”

Secretary of State John Kerry called the coalition vote “a big step forward and a significant one,” but did not immediately address how the conflicting demands would be resolved.

The United States and Russia had hoped to hold talks by year’s end, but no date has been set.

A version of this article appears in print on November 12, 2013, on Page A6 of the New York edition with the headline: Leading Syrian Opposition Group, Yielding to Pressure, Votes to Join Peace Talks. Order Reprints|Today's Paper|Subscribe